In mining, however, also in waste processing, for example, in the case of composting or in the case of decontamination treatments of the most varied of materials, a series of processes, especially chemical, biological processes, are performed using solids heaps. For example, in the treatment of the most varied of ores, especially in the mining of copper and iron ores, always more frequently, microbiologically supported, leaching processes are being used. In the case of this microbial leaching, which is also referred to as “bio-leaching”, microorganisms support the digestion of metal salts of insoluble ore minerals to water soluble salts. The terminology, microorganisms, refers, in the case of such processes, frequently to bacteria and archaea, which oxidize sulfide and elementary sulfur to sulfate and partially also divalent iron to trivalent iron. Examples of such bacteria include the sulfur bacteria, Acidithiobacillus ferrooxidans, which oxidizes sulfide, sulfur and iron, and Acidithiobacillus thiooxidans, which oxidizes sulfide and sulfur to sulfate. The processes involved in the case of microbial ore leaching can be aerobic or anaerobic. The oxygen required for this must be fed to the processes transpiring in the solids heap in sufficient amount, wherein, depending on whether the applied bacteria work aerobically or anaerobically, the supplied oxygen amount must be controlled to within a range suitable for the bacteria.
For example, known from published International Application No. WO 01/18269 A1 is a method for obtaining copper from copper sulfide minerals. In the case of this method, the mineral to be digested is slurried into a bio-leaching solution and the suspension stirred in a reactor sealed relative to the environment. Copper is dissolved out of the ore by microbiologically supported processes. By means of controlled ventilation, the slurry is fed a sufficient amount of oxygen. The control is based on an oxygen measurement within the reactor in the slurry and optionally in the gas phase.
Instead of in closed reactors, the microbial leaching can also be performed in a solids heap of piled, comminuted, hard rock ore. For this, the heap is sprayed from above with a leaching liquid, for example, with an aqueous sulfuric acid solution in a concentration of, for instance, 0.5 g/l. The microorganisms required for the process are located, in such case, within the heap on the surfaces of the hard rock ore or on their own nutrient substrate. While the leaching liquid trickles through the solids heap, microbial digestion of the difficultly soluble ore minerals occurs, so that metal ions, for example, iron or copper ions become enriched in the leaching liquid. The floor of the heap is sealed relative to the rock lying therebeneath, so that the leaching liquid enriched with metal ions can be taken off by means of channels and drains and collected. The collected leaching liquid can be sprayed back from above onto the heap. In given cases, it can earlier be subjected to an analysis, in order to test the efficiency of the metal leaching and in order, in given cases, to determine the acid consumption during the trickling of the leaching liquid through the heap. Corresponding to the ascertained acid consumption, acid can be added to the leaching liquid, before it is sprayed anew onto the heap. When the recirculated leaching liquid has become sufficiently enriched with the desired metals, the metal ions can be extracted, for example, by precipitation. The remaining, now metal ion poor, leaching liquid can, thereafter, be applied anew onto the heap. The ore leaching can last in this manner up to 150 days and achieve metal yields from the ore of up to 80%.
In order to provide sufficient oxygen for the microbial leaching process, the heap is aerated, for example, from its bottom, with oxygen rich air. In order to monitor and, in given cases, to be able to control (open or closed loop control) the aerating of the heap, a measuring of the oxygen concentration within the heap is required.
In the article, H. M. Lizama, Copper bio-leaching behavior in an aerated heap, Int. J. Miner. Process. 62 (2001), Pgs. 257-269, oxygen measurement within a solids heap is described, in the case of which gas samples are removed at different heights in the heap, wherein a steel tube is inserted down to desired depths of the solids heap and gas samples sucked from the different depths through the steel tube. Gas samples are fed to an oxygen gas sensor arranged outside the solids heap, and the oxygen concentrations in the gas samples measured.
Disadvantageous in such a method is, on the one hand, that it only delivers information concerning the oxygen concentration of the gas sample under the conditions reigning in the gas analyzer. No information concerning the locally (i.e. at the site of the sample taking) reigning oxygen partial pressure is provided. While, as a rule, standard conditions, such as a pressure of 1013 hPa, a standard temperature, e.g. 0° C., reign in gas analyzers, and the gas is usually dried before the measuring, there exists within the hard rock ore heap a pressure gradient resulting from the ventilation, increased temperatures resulting from the metabolism of the microorganisms and high humidity due to the leaching liquid trickling through the heap. Moreover, the sucking of the gas samples through the long steel tube can lead to delay and a “blurring” of the measurement results. The known method is, consequently, not in all cases precise enough.